Shallow water waveguide characterization using Hankel transforms—Preliminary experimental results

Abstract

Theoretical and computer simulation results recently obtained by Frisk and Lynch [G. V. Frisk and J. F. Lynch, J. Acoust. Soc. Am. 76, 205–216 (1984)] indicate that one can perform a relatively simple cw experiment which would, after processing of the data using the Hankel transform, yield a depth-dependent Green's function which contains information about the nature of the discrete and continuous modal spectra, as well as the plane wave reflection coefficients of the waveguide boundaries. Specifically, the method consists of measuring the magnitude and phase versus range of the pressure field due to a cw point source and then numerically Hankel transforming these data to obtain the Green's function versus horizontal wavenumber. In order to test this technique in shallow water, a series of experiments was performed in May 1984 at selected sites in Buzzards Bay and Nantucket Sound. The data collected at these sites appear to be of extremely high quality, and preliminary analysis using both forward modeling of the pressure field and the inverse Hankel transform (to obtain the Green's function and thus the modal structure) indicates that the method has considerable promise. Modal interference patterns in the pressure field and modal peaks in the Green's function are clearly observed. Of particular interest is the observation of a so-called “virtual” mode or “pseudoresonance” in the modal continuum, which we believe to be the first clear measurement of continuum structure in ocean acoustics. [Work supported by ONR.]